There are several types of sights available in the market to enable a user of optical devices or weapons such as rifles, shotguns, handguns, and submachine guns to aim these weapons. Examples of such sighting devices include laser sights, holographic sights, “reflex,” or “red dot” sights etc. Some of the available sights have a laser as their light source. The light may be used to create an image of a reflex or red dot. However, these images are susceptible to drift due to a change in the output wavelength of the laser source because of changing environmental conditions and may introduce error in aiming a weapon/optical device.
Laser diodes are used in a wide variety of applications that require a narrow spectral width. However, the wavelength of the light produced by the laser diode varies depending on a number of factors, including the temperature of the laser diode. For example, some laser diodes will exhibit a shift in output wavelength of approximately 0.30 nm/° C. The change in temperature of the laser diode may be due to environmental conditions or due to heating from operation of the diode. For some applications, this shift in wavelength is not a problem. However, for other applications, such as certain holographic gun sights, this shift in wavelength will cause the holographic gun sight to be inaccurate.
In typical a holographic gun sight, a holographic optical element (HOE) is illuminated by a reconstruction beam, from a laser diode, and reconstructs an image of a reticle as an object beam. The reconstruction beam typically does not illuminate the HOE perpendicular to the surface but instead is angled at a beam angle. Depending on how the HOE is made or recorded, the object beam is also at an angle to perpendicular. These angles are typically not equal (on opposite sides of perpendicular). In such a case, the actual object beam angle will vary depending on the wavelength of the reconstruction beam. This is called dispersion. As the wavelength of the reconstruction beam shifts, the diffraction angle from a holographic element will change, which will result in movement of the reconstructed holographic image and give an inaccurate reticle position relative to the target.
To correct for this change in wavelength, some sights are configured such that the system of holographic optical elements form an achromat to compensate for changes in wavelength. Another approach to addressing this problem is to control the temperature of the laser diode, such as through the use of a thermoelectric device or TEC cooler. However, this approach is not considered practical in low-power applications such as a gun sight. In applications where such control is practical, the control may be open or closed loop. An open loop control may be used, such as a temperature sensor attached to the laser diode. As the sensor temperature changes the TEC cooler will be adjusted. For a closed loop system, the wavelength output by the laser diode may be directly monitored. This information is then used to adjust the temperature of the laser diode via the thermoelectric cooler, and bring the diode back to the target wavelength. While thermal control of the laser diode is effective in preventing a change in wavelength, thermoelectric controllers are large in comparison to the laser diode and may draw a current in excess of 0.5 amps. For either case, using a thermoelectric cooler increases the physical size of the laser source assembly and greatly increases its energy requirements. For this reason, thermoelectric controllers in gun sights are impractical and undesirable at this time.